1. Field of the Invention
This invention relates to a coating system for an article, such as a syringe assembly, comprising curable organopolysiloxane(s), methods to reduce static and kinetic friction between slidable surfaces, and articles of low friction prepared thereby.
2. Description of Related Art
Certain devices require slow and controlled initiation and maintenance of sliding movement of one surface over another surface. It is well known that two stationary surfaces having a sliding relationship often exhibit sufficient resistance to initiation of movement that gradually increased pressure applied to one of the surfaces does not cause movement until a threshold pressure is reached, at which point a sudden sliding separation of the surfaces takes place. This sudden separation of stationary surfaces into a sliding relationship is herein referred to as “breakout”.
A less well known, but important frictional force is “breakloose force”, which refers to the force required to overcome static friction between surfaces of a syringe assembly that has been subjected to autoclaving and may have a slight deformation in one or both of the contacting surfaces of the syringe assembly, for example in the syringe barrel. In addition to autoclaving, parking of the assembly can further increase the breakloose force.
Breakout and breakloose forces are particularly troublesome in liquid dispensing devices, such as syringes, used to deliver small, accurately measured quantities of a liquid by smooth incremental line to line advancement of one surface over a graduated second surface. The problem is also encountered in devices using stopcocks, such as burets, pipets, addition funnels and the like where careful dropwise control of flow is desired.
The problems of excessive breakout and breakloose forces are related to friction. Friction is generally defined as the resisting force that arises when a surface of one substance slides, or tends to slide, over an adjoining surface of itself or another substance. Between surfaces of solids in contact, there may be two kinds of friction: (1) the resistance opposing the force required to start to move one surface over another, conventionally known as static friction, and (2) the resistance opposing the force required to move one surface over another at a variable, fixed, or predetermined speed, conventionally known as kinetic friction.
The force required to overcome static friction and induce breakout is referred to as the “breakout force”, and the force required to maintain steady slide of one surface over another after breakout or breakloose is referred to as the “sustaining force”. Two main factors contribute to static friction and thus to the breakout or breakloose force. The term “stick” as used herein denotes the tendency of two surfaces in stationary contact to develop a degree of adherence to each other. The term “inertia” is conventionally defined as the indisposition to motion which must be overcome to set a mass in motion. In the context of the present invention, inertia is understood to denote that component of the breakout force which does not involve adherence.
Breakout or breakloose forces, in particular the degree of stick, vary according to the composition of the surfaces. In general, materials having elasticity show greater stick than non-elastic materials, particularly when the surfaces are of dissimilar composition. The length of time that surfaces have been in stationary contact with each other also influences breakout and/or breakloose forces. In the syringe art, the term “parking” denotes storage time, shelf time, or the interval between filling and discharge. Parking generally increases breakout or breakloose force, particularly if the syringe has been refrigerated during parking.
A conventional approach to overcoming breakout has been application of a lubricant to a surface to surface interface. Common lubricants used are hydrocarbon oils, such as mineral oils, peanut oil, vegetable oils and the like. Such products have the disadvantage of being soluble in a variety of fluids, such as vehicles commonly used to dispense medicaments. In addition, these lubricants are subject to air oxidation resulting in viscosity changes and objectionable color development. Further, they are particularly likely to migrate from the surface to surface interface. Such lubricant migration is generally thought to be responsible for the increase in breakout force with time in parking.
Silicone oils are also commonly used as lubricants. They are poor solvents and are not subject to oxidation, but migration and stick do occur, and high breakout forces are a problem. Polytetrafluoroethylene surfaces provide some reduction in breakout forces, but this material is very expensive, and the approach has not been totally effective.
Thus there is a need for a better system to overcome high breakout and breakloose forces whereby smooth transition of two surfaces from stationary contact into sliding contact can be achieved.